1. Field of the Invention
The present invention relates to an optical head, and an apparatus for and a method of storing and reproducing optical information.
2. Description of the Related Art
An optical storage technique using an optical disk with a pattern of a pit shape, which is one of information storage media of high densities and mass storage volumes, is increasing its applications such as a digital audio disk, a video disk, a document file disk and further a data file. In the optical storage technique, the information is stored (recorded) and reproduced with high accuracy and reliability by applying a minutely condensed light beam onto the optical disk. The storing or reproducing action solely depends on the optical system. Fundamental functions of an optical head, which is a main portion of the optical system, are roughly divided into convergence for forming a minute spot of diffraction limit, focus control and tracking control of the optical system, and detection of pit signals. These functions are achieved by combining any one of various optical systems with any one of various photoelectric transferring and detecting methods in accordance with the object and use of the optical system.
On the other hand, in recent years, an optical disk of high density and mass storage volume, which is a called DVD, has been put to practical use and highlighted as an information storage medium which can treat a very large volume of information such as moving images. In the DVD optical disk, the pit size on the surface for storing information is reduced in order to increase its storing density in comparison with that of a compact disk (shortly referred to “CD” hereinafter) which is one of conventional optical disks. Accordingly, in the optical head for storing and reproducing information on the DVD optical disk, its wave length of the light source defining the spot diameter or its numerical aperture (shortly referred to “NA” hereinafter) of the convergent lens is different from that of the CD. In this connection, as for the CD, the wave length of the light source is set to about 0.78 μm while the NA is set to about 0.45. On the other hand, as for the DVD optical disk, the wave length of the light source is set to about 0.63-0.65 μm while the NA is set to about 0.6. Therefore, in order to store or reproduce the information for the two kinds of optical disks of the CD and the DVD using one optical disk drive, the optical head is required to have two optical systems. On the other hand, because the optical head is required to be small, thin and inexpensive, there is such a tendency that parts of the optical systems of the CD and the DVD are used in common to the best of their ability. For example, a technique may be used such that only one light source for the DVD is used while two kinds of convergent lenses for the DVD optical disk and the CD are used. Further, it may be also used such a technique that only one convergent lens is used in common, and then the NA is mechanically or optically changed such that the NA is set to a larger value when the DVD optical disk is used while the NA is set to a smaller value when the CD is used.
Meanwhile, in order to reproduce a CD-R which is rapidly and increasingly prevailing in recent years, there has been widely produced an optical head with light sources having wave lengths which are optimized for the DVD and the CD (CD-R), respectively. In FIG. 10, there is shown an example of an interchangeable optical head for reproducing the DVD and CD-R. In the optical head, the interchangeability between the DVD and the CD is achieved by using a convergent lens including a portion optimized for the DVD and another ring-shaped portion optimized for the thickness of the CD, and by using a light source for the CD and another light source for the DVD.
Hereinafter, the optical head will be described with reference to drawings. As shown in FIG. 10, in the optical head, on an optical path between a disk 8 (i.e., DVD 8a or CD 8b) and a photo detector 54, there are disposed an objective lens 7, a collimator lens 3, a rising mirror 9 and beam splitters 5, 6. A light source 51 for reproducing the DVD is disposed on an optical path which is diverged from the beam splitter 5 while another light source 52 for reproducing the CD is disposed on an optical path which is diverged from the beam splitter 6. In the optical head, the objective lens 7 has a shape to be able to form a spot having a size which is optimized for both of the DVD 8a and the CD 8b, the thickness of the base material of the DVD being smaller, and the thickness of the base material of the CD being larger (see FIGS. 5A and 5B). Meanwhile, the beam splitter 5 is composed of a plate having surfaces parallel to each other (parallel plate). As for both of a light beam 62a for reproducing the DVD 8a and a light beam 62b for reproducing the CD 8b, the reflected light beams of those generate astigmatism in the beam splitter 5. The DVD 8a is reproduced while composing the photo an detector 54 of a quartered detector and using the so-called astigmatism method as the focus error detecting method. Hereupon, as the tracking detecting method, the so-called phase difference method is used if the DVD 8a is a DVD-ROM while the push-pull method is used if the DVD 8a is DVD-RAM. In this connection, as the tracking method for the CD 8b, the so-called three beam method is generally used using diffracted light which is generated by a grating (diffraction grating) 60 disposed between the light source 52 for the CD and the beam splitter 6. Moreover, in order to stabilize the push-pull signals of the DVD-RAM, it is also suggested to use the so-called differential push-pull (DPP) method using diffracted light which is generated by a grating 59 disposed between the light source 51 for the DVD and the beam splitter 5, as same as the case of reproducing the CD 8b.
However, in the conventional interchangeable optical head using the respective light sources for the DVD and the CD, it is required to provide at least two beam splitters. Meanwhile, if different tracking error detecting methods are used for the DVD and the CD, respectively, namely if the DPP method is used for reproducing the DVD while the three beam method is used for reproducing the CD, it is required that respective optical elements with diffraction gratings are provided for both of the DVD and the CD. Therefore, the above-mentioned matters prevent the optical head from being made smaller or more inexpensive.